the layers were separated. The aqueous layer was neutralized with sodium bicarbonate and extracted with three 50-mi portions of ether. The combined ether extracts were concentrated at reduced pressure on the steam bath.18 If the residue contained a mixture of solid and an oil, it was washed with 10 ml of cold (0°) methanol and filtered immediately. If a solid was not (18) Conveniently, crystallization was found to be hastened if the ether extracts were not dried before removal of excess solvents, especially when p-methoxyaryl isoxazoles were synthesized.Kovacs, Mayers, Johnson, Cover, and Ghatak formed, 5-10 ml of methanol was added and crystallization occurred upon refrigeration of the mixture. Recrystallization was effected with ethanol or benzene (see Table I).

The relationships between the response of the vibrating dropping mercury electrode (VDME) and fundamental electrode parameters such as drop time, rate of mercury flow, and frequency of vibration are examined. Close correlations with polarographic theory are found. Conformance to the llkovic equation by a mass-transport-controlled wave is observed over a wide range of experimental conditions down to millisecond drop times but the polarographic criterion for a diffusioncontrolled wave is inapplicable to the VDME. The current-potential curve for a reversible electrode process obeys existing theory over the entire range of conditions examined.

the qualitative description of the electrode processes involved in the operation of liquid membrane electrodes cannot yet be made quantitative. Accurate measurements of a high impedance over a wide frequency band-width are extremely difficult to carry out, but it seems likely that refinements and improvements of the present technique can lead to a possible means for the quantitative evaluation of fundamental electrode parameters. ACKNOWLEDGMENT Special experimental facilities were provided by the University Program for Scientific Measurement and Instrumentation.

No abstract

Equation 3a represents the mass transport step, Equations 3b and 3c are equilibria involved in the adsorption of the uncharged PQP molecule, Equation 3d represents the overall electron transfer step (not balanced), and Equations 3e and 3f indicate that the adsorbed quinone is desorbed and diffuses into the bulk of the solution.The peak voltammetry experiments most conclusively indicate that adsorbed PQP participates in the charge transfer step. For an adsorption postwave to occur, PQP must not desorb from the electrode surface in the time required to sweep the potential through the oxidation wave. Because the post-wave is first observed at ca. 1 V/sec, this time is in the range of 5 X 10-2 to 5 X 10~1 234 56789sec.This scheme is not meant to imply that the oxidation pathway must proceed via an adsorbed species. Indeed, in the potential region of diffusion control, the concentration of PQP at x = 0 will be zero, Equation 3c will be driven to the left, and PQP which reaches the electrode by diffusion will be oxidized at x = 0. Thus the oxidation process is distributed between two pathways. A subsequent paper will discuss the effect of the adsorption equilibria on the electrochemical oxidation of PQP at carbon paste electrodes.